This invention relates to directional doppler ultrasound systems for biosignal detection and acquisition. As used herein the term biosignal means a signal produced by or from or derived from a physiological phenomenon, i.e. a biophysical event.
Doppler systems, particularly doppler ultrasound systems, are commonly used to detect biophysical events such as blood flow, heart motion and fetal heart motion. Certain applications (e.g. fetal heart motion) involve detection of a multiplicity of doppler components arising from the desired organ's motion which may then be used to count the rate of occurrence of events or as a time reference for physiologic measurements (e.g. calculating time intervals). Unfortunately, doppler components all too often are present which result from motions of other objects of no interest but which are nevertheless in the field "viewed" by the system. This factor, coupled with the fact that the desired organ (e.g. the heart) may itself contain several moving structures, moving in different directions at different velocities, at the same time as well as in sequence, usually results in the reception of a rather complex composite signal.
In the present art, these complex signals are used directly, and various processing methods such as filtering, resonant circuits, etc. may be used to attempt to reduce the complexity of signals representing a complex event (e.g. a heart beat) to a single pulse. However, in addition to the fact that present doppler arrangements are at times unable to reliably provide from such complex signals the accuracy desired in the medical profession regarding for example interval measurement, there is need to single out certain portions of the signal representing reproducible, predictable, unique physiological events to allow measurement of inter-event details (e.g. in regard to the heart pre-ejection period, left ventricular ejection time, isometric relaxation time, etc.) and there is the need, particularly in monitoring the fetal heart, for singling out single physiologic phenomena to render inter-event counting (e.g. heart rate) more accurate.
Monitoring the heart rate of the fetus is one application of special importance. Here, there is great interest in accurate counting of each beat-to-beat interval (time between heart beats) by doppler ultrasound, and in addition, there is great interest in measuring the time from the fetal ECG to given valve motion in the fetal heart.
More particularly in regard to fetal monitoring per se, it is for example believed by many that short term fetal heart rate (FHR) variability patterns (beat-to-beat changes) may contain information concerning fetal wall being. In addition to their part in forming the long term variability patterns, they may represent the effects of fetal respiratory efforts, the minute adjustments of blood volume in the fetal-placental unit, changes in fetal blood pressure and the effects of drugs. These patterns are of particular interest in antenatal fetal evaluation.
Accurate measurement of RR (R-wave to R-wave) interval variation is known to be easily accomplished after rupture of membranes by the direct (or so-called internal) measurement approach of fetal scalp electrocardiography because the signal for each heartbeat is unique, bold, usually has a high signal-to-noise ratio, is stable and always represents the same physiological event.
In contrast, it is well known that the abdominally (indirectly) derived fetal ECG (i.e. AFECG), though related to a unique physiological event, is not really amenable to beat-to-beat recording because it is frequently obscured by the maternal ECG; its signal-to-noise is often very poor or low and it is unobtainable in over 50% of cases in addition the signal is very difficult to obtain in the critical 32nd to 36th week of gestation. While it has appeal as a potential method of accurate external (to the mother) fetal RR interval measurement, these engineering and physiological considerations prevent the abdominal fetal ECG from realistically approaching true RR interval measurement (as obtained for example through an internal monitoring approach) and therefrom deriving accurate RR interval change or variability.
Moreover, these same considerations prevent the AFECG from realistically approaching the so-called external (ultrasound) doppler in overall ability to obtain a usable record. This is demonstrated for example by the fact that the external doppler system is able to produce heart signals in virtually all patients (fetuses) and is not encumbered by the maternal heart signal, thus rendering its potential for accurate external counting great.
Original research in physiology has demonstrated the cardiac events which compose the complex doppler returns from the fetal heart, and their inter-relationship, and their relation to fetal heart rate. This has led to selection of optimal doppler frequencies and design of patented means for processing (see for example, U.S. Pat. No. 3,763,851 to Hatke et al. and U.S. Pat. No. 3,934,577 to Romani). In the present art the three major doppler components of fetal heart motion, i.e. artrial wall motion, aortic and A-V valve closing, and A-V valve opening, are all presented for counting and listening. These signals include and represent, however, motion of valves, etc. toward and away from the system's transducer arrangement.
As a general consideration, it would be most useful to simplify the doppler signal presented for counting or measurement at the outset by selection of the doppler signal(s) of interest by known physiological characteristics. In the present art, attempts have already been made to simplify the doppler signals for example by use of narrow beam transducers, which allow manual selection of events by careful aiming. This does not eliminate confounding signals in front of or behind the region of interest, or signals from objects moving in the opposite direction.
There exists presently a fetal monitoring system employing ranging techniques which enables substantial elimination of some of the confounding signals such as the doppler signals developed outside, i.e. in front of or behind, the immediate area of interest. It is known, for example, that a ranging-autocorrelation doppler system closely approximates the long-term and to some extent the short term variability obtained by fetal scalp electrocardiography. Such a system is able to provide perhaps a 10db (three-fold) signal enhancement in areas of interest and 30db reduction in other areas. This increases the signal-to-noise ratio and renders accurate counting more feasible.
However, even in the immediate area of interest such as the fetal heart itself, there are a number of movement events occurring naturally for each heart beat (e.g. the atrial motion, AV-valve closings and openings etc.) which give rise to a composite doppler signal which remains complex even with narrow beam transducer operation and ranging techniques, not to mention filtering and other prior art attempts to simplify the composite signal for purposes of, for example, accurate counting.